ARRAN. GEOLOGY. GRANITE. 349 



the numerous varieties which this substance presents; 

 it will be sufficient to notice the most remarkable. Some 

 of these are mixtures of the three most common ingre- 

 dients, quartz, felspar, and mica, while others are charac- 

 terized by the presence of hornblende. One of the most 

 remarkable peculiarities which it displays, is a tendency 

 to decompose in crusts parallel to a central mass ; a phe- 

 nomenon common in the trap rocks, and which has been 

 supposed to indicate similar circumstances in the origin 

 of both. Another peculiarity may also be occasionally 

 seen in the granite of this island, namely, a circular dis- 

 tribution of the mica. This sometimes occurs in granite 

 veins, and is connected with a considerable number of 

 appearances in rocks, the most remarkable instance of 

 which is found in the orbicular granite of Corsica. A 

 similar distribution is to be seen in some parts of the 

 Serpentine of Anglesea. 



The texture of the granite of Arran is sometimes coarse, 

 at others fine ; the latter variety occupying the western 

 side of the island, while the former is limited to the 

 eastern : it is unnecessary to draw a more accurate line. 

 In some cases the coarse varieties contain distinct crystals 

 of felspar imbedded in the more regular granitic basis; 

 an appearance common in the granite of Cornwall, and 

 from which this variety has been termed porphyritic. 

 This granite, of whatever texture, is generally grey, the 

 felspar being whitish and the mica black : but a red 

 variety occurs in small quantity on the north side of 

 Glen Sannox. In some cases the general grey mass is 

 stained with yellowish brown, and blackish spots, com- 

 monly very distinct, and producing a peculiar effect. This 

 stain seems to be foreign to the rock, and chiefly to 

 pervade the felspar; yet it does not seem the effect of 

 any oxydation of a metallic matter, as it is found in the 

 central parts of fresh fragments. It has the appearance 

 often given by manganese to sandstones. Another pecu- 

 liarity is not unfrequent in the granite of Arran, namely, 



